Expanding the Vision of Sensor Materials (1995)
Chapter: APPENDIX G: CANDIDATE SENSOR TECHNOLOGIES FOR DETECTION OF CHEMICAL WEAPONS...
APPENDIX G
CANDIDATE SENSOR TECHNOLOGIES FOR DETECTION OF CHEMICAL WEAPONS (SCHEDULE II COMPOUNDS)
Candidate technologies have been identified by attempting to match the descriptors presented in Table 6-3 to available sensor systems. For the sake of clarity, only fatal shortcomings (indicated by *) and other descriptors deemed worthy of particular consideration are included in the following discussion. Additional descriptors may be necessary for detailed description of the status of development and the cost to implement the technology.
Gas chromatography-mass spectrometry systems currently provide the standard for sensitivity measurements and will fulfill requirements. However, as indicated in Table G-1, existing systems are too large, heavy, and expensive to meet needs.
Current ion mobility spectrometer systems can only detect part-per—million levels of very electronegative or electropositive species. These systems are not sufficiently sensitive for treaty verification, although they could be useful in battlefield conditions. The comparison of several available attributes for these systems to the requirements is shown in Table G-2.
Patch-test chemical reactions exploit selective chemical reactions that are irreversible and result in color changes. They are reported to be sensitive to selected Schedule II compounds in the parts-per-billion range. Their response time is slower than required, as shown in Table G-3.
Fiberoptic micromirrors use reliable communications technology to measure reflectance of sensor coating on the end of a cleaved fiber. The reflectivity can be changed by the analyte, by changes in the thickness of the sensing film, or by a change in refractive index. The sensor is very small and light and is immune from electrical interference. A poly-siloxane film has been shown to respond to di-isopropyl-methylphosphonate (DIMP).
TABLE G-1 Key Descriptors for Gas Chromatography-Mass Spectrometry
|
Descriptor |
Requirements (Specification) |
Available (Attributes) |
|
Constraints-packaging (size, weight) |
Hand held, volume < 1 cubic foot, weight 2.3 kg (5 lb). |
Too large and heavy; survey mobile analyzer on two-wheeled cart weighs ~ 80 kg. |
|
Economic-acquisition |
Unit cost ~ $10,000. |
~$50,000 for portable mass spectrometer; $130,000 for rugged, portable GC-MS. |
|
Characteristics-response time |
< 1-minute response and recycle. |
Can be as low as 1 minute. |
TABLE G-2 Key Descriptors for Ion Mobility Spectrometer
|
Descriptor |
Requirements (Specification) |
Available (Attributes) |
|
Constraints-packaging (size, weight) |
Hand held, volume < 1 cubic foot, weight 2.3 kg (5 lb). |
Hand held unit resembling toolbox, ~1 foot long, weighing 20 kg (44 lb), with no vacuum pump required. |
|
Characteristics-response time |
< 1 minute response and recycle. |
1 minute. |
TABLE G-3 Key Descriptors for Patch-Test Chemical Reactions Showing Color Change
|
Descriptor |
Requirements (Specification) |
Available (Attributes) |
|
Characteristics-response time |
< 1-minute response and recycle. |
About 1 minute, but unit is nonreusable. |
TABLE G-4 Key Descriptors for Fiberoptic Sensors
|
Descriptor |
Requirements (Specification) |
Available (Attributes) |
|
Characteristics-response time |
< 1-minute response and recycle. |
Approximately 1 minute at low concentrations. |
|
Characteristics-sensitivity |
Approximately 1 part per billion in gas phase. |
1 ppm of DIMP. |
|
Characteristics-resolution |
Approximately 1 part per billion in air. |
Change in reflectivity of 0.01% can be observed with prototype apparatus, corresponding to ~1 ppm of DIMP. |
|
Characteristics-range |
From 1 thousand to 1 part per billion. |
Large dynamic range observed: 1 ppm to 100 part per billion for DIMP. |
|
Characteristics-limit of detection |
1 part per billion. |
Approximately 1 part per million using polysiloxane film. |
|
Characteristics-selectivity |
*High. |
Selectivity is poor; sensors respond to most organic solvents. |
|
Characteristics-accuracy |
Detection at 1 part per billion level may be sufficient. |
± 10 percent. |
|
Economic-acquisition |
*Unit cost of ~$10,000 acceptable. |
No commercial units available. |
|
Economic-development |
Estimate that up to $10 million could be spent over 10 years to develop sensor system. |
Considerable development of coatings required for chemical selectivity. |
Table G-4 contains a comparison of requirements versus available technology. This technology currently suffers from low selectivity and high cost.
Electrochemical sensors are available for chemical analysis, including ion selective electrodes for solvated (usually in water) analytes, amperometric analysis for both gas and liquid phases, and biosensor systems. Electrochemical sensors are amenable to array designs. The chemical selectivity issues are similar to those for fiberoptics, as depicted in Table G-4.
The basic technology for surface acoustic wave (SAW) sensors involves the use of small, thin plates of a piezoelectric material, often quartz, with evaporated metal electrodes for applying radio frequency signals (see Appendix F). Chemically sensitive films on the piezoelectric material have their properties altered by the adsorption of molecules of interest. Often the mass of adsorbed molecules is measured, but subtle changes in the mechanical and electrical properties of the films are also detected. SAW sensors can exhibit high sensitivity; in some cases a thousandth of an adsorbed monolayer of analyte can be detected. Considerable research and development has gone into developing SAW technology, notably highly selective chemical coatings, for sensitive and selective sensors for nerve agents and their precursors and degradation products. The technology is amenable to array designs for pattern recognition. Table G-5 compares requirements versus available technology. This technology currently suffers from variable selectivity.
TABLE G-5 Key Descriptors for Acoustic Wave Chemical Sensors
|
Descriptor |
Requirements (Specification) |
Available (Attributes) |
|
Characteristics-response time |
< 1-minute response and recycle. |
1-10 seconds, reversible. |
|
Characteristics-sensitivity |
Approximately 1 ppb in gas phase. |
1 ppm commonly, potentially 1 ppb with preconcentration hardware. |
|
Characteristics-range |
From 1 part per thousand to 1 part per billion (ppb). |
Good dynamic range. |
|
Characteristics-limit of detection |
1 ppb. |
Potentially approximately 1 ppb. |
|
Characteristics-selectivity |
*High. |
Highly variable, but promising in a few cases. |
|
Characteristics-accuracy |
Detection at 1 ppb level may be sufficient. |
Frequency shifts can be read to 1 part in 109, which translates to 1 part per million out of a signal from a concentration of 1 part per thousand. |
|
Constraints-packaging (size, weight) |
Hand held, volume < 1 cubic foot, weight < 5 lbs (2.3 kg). |
With electronics, about the size of a pager; complete system (including computer for data acquisition) potentially suitcase sized; weight 5-10 kg for entire portable system. |
|
Economic-development |
Estimate that up to $10 million could be spent over 10 years to develop sensor system. |
Some custom units are now available; there is substantial development research activity. |
TABLE G-6 Key Descriptors for Sensors Using Immochemical Assays
|
Descriptor |
Requirements (Specification) |
Available (Attributes) |
|
Characteristics-response time |
< 1 minute response and recycle. |
10 minutes to 2 hours; involves wet chemical steps; unattended, fieldable assay that can be remotely placed does not exist. |
|
Characteristics-sensitivity |
Approximately 1 ppb in gas phase. |
Parts per trillion in best laboratory-based design. |
|
Characteristics-resolution |
Approximately 1 ppb. |
Can be ~1 ppb. |
|
Characteristics-range |
From 1 part per billion to 1 part per thousand. |
Can be designed for parts per trillion, or any higher level. |
|
Characteristics-limit of detection |
1 ppb. |
Parts per trillion. |
|
Characteristics-selectivity |
High. |
Extremely high; potentially a drawback, since slight changes in chemical structure of agent being manufactured could make kit obsolete. |
|
Constraints-packaging (size) |
Volume < 1 cubic foot. |
Threshold assay field units can consist of credit card sized platform & several cigarette sized reagent vials; more precise assays require larger unit. |
|
Economic-development |
*Unit cost ~$10,000. |
~$50,000-100,000 for antibodies; and $100,000 for assay kit development. |
Immunochemical assays1 offer high selectivity and sensitivity through the use of antibodies raised against specific nerve agents or classes. (Many different types of antibodies are now available from many biotechnology companies.) The assay sensor uses these antibodies in one of several schemes in which the attachment of the antibody to the target molecule (e.g., nerve agent) results in a series of chemical reactions that produce a detectable change in the final sample. As indicated in Table G-6, the main shortcomings of the current generation of these sensors involves slow response time and high cost.
